During a bonding process, the pressure distribution on the wafers which are to be welded is of immense importance. The pressure distribution is a direct property of the surface waviness and surface roughness of the contacting surfaces of all tools which lie over or under the wafer. The tools include mainly heaters, clamping plates, compensation plates, receivers (bond chucks), etc. The knowledge of the surface waviness and surface roughness of the tool whose surface is in direct contact with one of the wafers is not sufficient for computing the pressure distribution. Therefore with ordinary measurement methods such as AFM, laser interferometry, white light interferometry it is not possible to obtain direct and reliable information about the pressure distribution of the bonding process. These measurement methods are used solely and alone for determining the topography of the surface of the tool which is directly in contact with one of the wafers which are to be welded. The homogeneity of the pressure distribution has a very great influence on the bond quality, from which the scrap rate of completely bonded wafers and the scrap rate of the functional components which are located on the wafer follow. Different adverse consequences in the case of insufficient uniformity of the pressure and/or of uneven tools will be briefly mentioned here. In the case of bonding interfaces which cannot be deformed or which can hardly be deformed, for example in metal-metal bonds, an inadequate pressure uniformity leads to poor bonding or no bonding at all at those locations on which the pressure was too low. For example Cu—Cu, Au—Au, Al—Al, W—W, Ni—Ni or Ti—Ti bonds are mentioned.
For moderately deformable bonding interfaces, for example eutectics which pass through a liquid phase during the bond process, a non-uniform pressure distribution can lead to an inhomogeneous structure. For very easily deformable bonding interfaces, for example temporary bonding adhesives, the non-uniform pressure distribution at the start of the bonding process leads solely to an inhomogeneous distribution of the layer thickness of the bonding adhesive.
To determine the pressure distribution, at present in industry mainly pressure films are used whose material reacts sensitively to pressure loading. The material changes color, depending on the corresponding pressure, to different degrees, or changes the intensity of its color. An optical evaluation of the color change or intensity change allows conclusions about the pressure distribution which would act on a perfect wafer which is located between the tools. The pressure films can only be used at room temperature.
The surface quality of all tools which are fixed on top of one another decreases in a plurality of bond steps so that from time to time repeated measurement of the pressure distribution is necessary to minimize the scrap, especially in wafer bonding. The wear is detectable mainly in plastically deformable materials like metal. At an unfavorable pressure distribution which can no longer be tolerated, in the worst case all tools which are fixed on top of one another must be reworked.
Alternative measurement methods for determining the pressure distribution work with force transducers with which however only a very coarse resolution along the receiver or the clamping plate is possible due to the size of the force transducers.